Coupons: Sites like RetailMeNot and MyCoupons.com are great, but that data should not be singled out in a standalone site. It should be integrated into standard search results (like Yahoo Shopping).

Price Alerts: Obviously, if I am truly searching for the best deal, I want to be informed when it happens. The tool to set up price alerts should be prominently displayed. PriceSpider does a good job of this. Sites like Become.com even offer price drops via email without registration. On the other hand, sites like Shopzilla don't even appear to have the feature—or they hide it behind a registration form. Of course, there are numerous sites out there like ZooAlerts, PricePinx and Camel Camel Camel that specialize in price alerts, but I don't see much value in those—especially when they are focused entirely on one shopping site like Amazon.

Price Trending: This is one of the new features on the block. Some comparison engines like NexTag and PriceSpider have already done a good job of integrating this feature in with search results, and it could help consumers rate the quality of the current deal by comparing it to prices in the past. Again, there are standalone sites like Gazaro that specialize in this feature, but as comparison site expert Brian A. Smith from comparisonengines.com points out, focusing entirely on price alerts and/or price trending is not an ideal strategy:

Comparison search engines only show you whose price is best in that instant. Now, at least one…
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In this economy, any site that can help a consumer find a great deal is going to get some attention, and I think price tracking is a smart concept, but it's nothing new. While the sites you mentioned: Gazaro, Zoolert, and PriceSpider have jazzed things up a bit with a web 2.0 look, price tracking has been available on shopping comparison engines (aka price comparison engines) like NexTag and PriceGrabber for a long time. I think that price alerts are a simple feature. I don't think there is enough meat there to make a real product or business. If you look at Gazaro and Zoolert versus PriceSpider, you'll see that PriceSpider is generating much more traffic. I think this is partly because PriceSpider has ventured beyond just price alerts to shopping comparison engine listings.

He also offers a warning:

Just because a price tracking site shows you a seemingly great new alert, the buyer should always beware. Most price tracking sites that I've looked at do not have a deep depth of merchants, but are rather just joining some select affiliate programs through Commission Junction or Linkshare. Because of this, a price drop from one merchant might look impressive, but in the end might not actually provide a consumer with a great deal.

A Clean, Usable UI: With so many details to keep track of, it's not easy to keep things clean. Personally, I have never been a fan of NexTag's layout—it seems kind of text heavy and convoluted to me. On the other hand Shopzilla has taken a more Web 2.0 approach while Google Product Search stays true to the Google design mantra. In the end, this is really a matter of preference.

User and Expert Reviews: Another no-brainer. Again, these should be prominently displayed with the product.

The bottom line is that on their own, the tools listed above give you only part of the picture—but when used together they can be extremely valuable to consumers. None of the websites I have come across are doing everything right—but I feel that sites like NexTag and PriceSpider are headed in the right direction as far as features are concerned while sites like PriceGrabber, Shopzilla and Yahoo Shopping are still tops in terms of overall effectiveness—a sentiment echoed by our expert from comparisonengines:

Consumers should make sure to take a look at a shopping behemoth like Shopzilla or Yahoo! Shopping before making a purchase. Yahoo! Shopping has an extremely comprehensive deal section, and even better, the site integrates coupons right into shopping comparison engine listings so consumers will have greater transparency into the deal (see the listings for Crutchfield, Tiger Direct, and ABT). So using a site like Yahoo! Shopping provides the consumer with a greater number of merchants, a shopping comparison engine experience (sort by price, rating, etc.), and integrates coupons.

Until one site puts all of the pieces together, it will still be necessary to check multiple websites to ensure that you are getting the best deal online. Hopefully, my rant on comparison engines will, at the very least, help you narrow down the search to save both time and money.

Prof. Dealzmodo is a regular section dedicated to helping budget-minded consumers learn how to shop smarter and get the best deals on their favorite gadgets. If you have any topics you would like to see covered, send your idea to tips@gizmodo.com, with "Professor Dealzmodo" in the subject line.